The present disclosure relates generally to electronic memory technology, and more specifically to the introduction of an in-situ annealing operation to improve the tunneling magneto-resistance (TMR) of magnetic tunnel junctions (MTJs).
Spin transfer torque magnetic random access memory (STT-MRAM) is an attractive emerging memory technology, offering non-volatility, high performance and high endurance. A typical STT-MRAM includes a multi-layered MTJ memory cell in series with a field effect transistor (FET), which is gated by a word line (WL). A bit line (BL) and a source line (SL) run parallel to each other and perpendicular to the WL. The BL is connected to the MTJ, and the SL is connected to the FET. One MTJ memory cell along the BL is selected by turning on its WL. When a voltage (e.g., 500 mV) is forced across the cell from BL to SL, the selected cell's MTJ is written into a particular state, which is determined by the polarity of this voltage (BL high vs. SL high).
Crystalline lattice structures can have special electrical properties that polycrystalline and amorphous lattice structures normally cannot. Forming a tunnel barrier layer of the MTJ from a crystalline material such as MgO results in a higher TMR. Providing a large TMR is desirable because a large TMR allows electrons to more easily tunnel from one ferromagnetic layer (e.g., an MTJ free layer) through the thin dielectric tunnel barrier into the other ferromagnetic layer (e.g., an MTJ fixed layer). Thus, a larger TMR results in a larger difference between the MTJ free layer resistance and the MTJ fixed layer resistance, thereby improving the ability to read the MTJ state (e.g., a “0” or a “1”).
TMR may be driven even higher by extending the crystalline lattice structure of the MgO tunnel barrier into the ferromagnetic free layer and the ferromagnetic reference layer that are adjacent to the MgO tunnel barrier. More specifically, an even larger TMR is expected when the lattice structures of the MTJ ferromagnetic layers (e.g., the free layer and the reference layer) crystallize into the body-centered-cubic (bcc) (e.g., bcc (100)) texture and are lattice matched to that of the MgO tunnel barrier. Because MTJ ferromagnetic layers are amorphous in their as-grown state, a post-deposition annealing step is needed in order to crystallize the MTJ ferromagnetic layers that are adjacent to the MTJ tunnel barrier.